Electromagnetic loudspeakers utilizing a vibrating diaphragm as a sound generator have existed previously in various forms. Although the magnetic structures of such speakers have varied considerably, numerous problems have been encountered.
In Gamzon, U.S. Pat. No. 3,013,905, the magnetic structure includes ceramic magnets which are of considerable size and cannot be materially reduced in size. As a result, the magnetic zones must be widely spaced from one another and the conductors on the diaphragm must also be identically spaced from each other. As a result, the driving forces applied to the diaphragm cannot be maximized.
In the Winey U.S. Pat. No. 3,674,946, the permanent magnet is of a rubber bonded barium ferrite composite material wherein the ferrite particles are mechanically oriented during processing and bonded in the rubber or plastic matrix or binder to hold them in place. The specific magnetic material is known by its trademark PLASTIFORM, sold by 3M Company of Saint Paul, Minn. This magnetic material is formed in sheets which may be apertured so that the magnetic material is acoustically transparent, the material may also be cut in strips as disclosed in the patent and as subsequently illustrated in a later U.S. Pat. No. 3,919,499. Such PLASTIFORM material and other flexible magnetic material such as that made under the trademark KOROSEAL by B. F. Goodrich Company, Akron, Ohio, do not have sufficient coercive force as may be desired in such speakers under some circumstances.
Another U.S. Pat. No. 3,873,784, Doschek, discloses a transducer with a diaphragm and utilizing a permanent magnet of an alloy of iron, nickel, aluminum or cobalt, and preferably a sintered ferrite material. Such sintered material is extremely difficult to work with and cannot easily be used in small pieces, thereby encountering the same problems as in Gamzon patent above.
Recently, magnets with extremely high coercive force have been produced in sizes that may be suitable for use in the magnetic structures of diaphragm type loudspeakers. Such magnets include polymer molded samarium cobalt magnets. Such samarium cobalt magnets are also sintered instead of being carried in a polymer binder. Such new materials have a much higher coercive force and magnetic flux density than previously available materials, but such new materials are extremely expensive, especially for use in diaphragm type speakers wherein broad areas of diaphragm must be accommodated.